Vol. 297, Issue 3, 1152-1159, June 2001

Acetylcholine-Induced Desensitization of the Contractile Response to Histamine in Guinea Pig Ileum Is Prevented by Either Pertussis Toxin Treatment or by Selective Inactivation of Muscarinic M₃ Receptors

Darakhshanda Shehnaz, Khurram Z. Ansari and Frederick J. Ehlert

Department of Pharmacology, College of Medicine, University of California, Irvine, Irvine, California

We have studied the role of M₂ and M₃ muscarinic receptors in acetylcholine-mediated desensitization of the contractile response to histamine in the guinea pig ileum. Treatment of the isolated ileum with acetylcholine (30 µM) for 20 min caused a marked desensitization of the contractile response to histamine. When measured 5 min after washout of acetylcholine, the EC₅₀ value of histamine increased 5.8-fold compared with that estimated before acetylcholine treatment, whereas the maximal response was unaffected. This shift in the EC₅₀ value of histamine was maximal at the earliest time measured after acetylcholine treatment (5 min), and normal sensitivity recovered in approximately 20 min. Acetylcholine-induced desensitization was prevented by uncoupling of M₂ receptors from G_i with pertussis toxin or by selective inactivation of M₃ receptors with N-2-chloroethyl-4-piperidinyl diphenylacetate (4-DAMP mustard). The shifts in the EC₅₀ values of histamine measured 5 min after acetylcholine treatment were only 2.0- and 1.8-fold in pertussis toxin- and 4-DAMP mustard-treated ilea, respectively. Both pertussis toxin- and 4-DAMP mustard-treatment had little or no effect on histamine-induced contractions in control ileum. Measurement of histamine-stimulated inositol phosphate accumulation in the longitudinal muscle of the ileum showed little or no inhibitory effect of prior exposure to acetylcholine, indicating that the majority of the heterologous desensitization occurs downstream from phospholipase C activation. Collectively, our results suggest that activation of both M₂ and M₃ receptors is required for heterologous desensitization of histamine-mediated contractions in the guinea pig ileum.